As a substrate for mounting an electronic device such as a SAW filter or the like, a via substrate is used. The via substrate has the structure of providing through holes in an insulating substrate such as ceramics and of filling the through holes with a conductor to form through electrodes. Recently, as communication devices, such as mobile phones, are miniaturized, it is demanded to miniaturize, and reduce a height of, an electronic device used therefor. The via substrate forming a part of the device is also required to be thinned.
Further, as to a diameter of the through hole of such substrate, it is demanded reduction of a size of the hole to 100 μm or smaller, for example, and it is further demanded to form many holes at a high density. Further, as a material of the supporting substrate, a high resistance is required for suppressing leak current between wirings.
Therefore, it is demanded that the via substrate has a strength for enduring handling of it during production process in the case it is thinned. On the other hand, since the via substrate is finally divided into individual pieces by dicing, its processability is required to be good.
Since through electrodes of the via substrate are produced by a thin film-formation process such as vapor deposition, sputtering or the like, it is required that the upper face, bottom face and inside of the through hole are smooth, for avoiding disconnections or the like due to unevenness. The through holes are formed by pressing using a metal mold and laser processing before the sintering, or by laser processing or blast processing after the sintering (Patent documents 1 to 3).
As a material of the via substrate, alumina ceramics has been used, because of excellent mechanical strength, heat resistance, insulating property or the like. Further, an alumina ceramic material of a high purity is superior in transparency and resistances to alkali and plasma, it has been widely used for a luminous vessel for a high-luminance discharge lamp, a member for a semiconductor production system and a dummy wafer (patent documents 4 to 7).